Vapor diffusion coating containing aluminum-chromium-silicon



Jan. 23, 19-68 PUYEAR ET AL 3,365,327

NG CONTAINING ALUMINUM-CHROMIUMSILICON VAPOR DIFFUSION COATI Filed April14, 1965 2 Sheets-Shqet l HOLLOWAY ROBERT B. PUYEAR JACK H A r TORNEVJan. 23, 1968 R. B. PUYEAR ETAL 3,365,327

VAPOR DIFFUSION COATING CONTAINING ALUMINUM-CHROMIUM-SILICON Filed April14, 1965 2 Sheets-Sheet 2 INVENTORS JACK H. HOLLOWAY ROBERT B.PUYEARATTORNEY United States Patent Ofilice 3,365,327 Patented Jan. 23, 19 683,365,327 VAPOR DIFFUSION COATING CONTAINING ALUMINUM-CHROMIUM SILICONRobert B. Puyear, Kokomo, Ind., and Jack H. Holloway,

Marshalltown, Iowa, assignors to Union Carbide Corporation, acorporation of New York Filed Apr. 14, 1965, Ser. No. 448,082 4 Claims.(Cl. 117-1072) A method in accordance with the present invention forproducing a coated metallic article comprises embedding the articles tobe coated in a particulated mixture of elemental chromium, elementalaluminum, silicon carbide and halide carrier material; and heating themixture and embedded article with the exclusion of air at an elevatedtemperature whereby a diffused metallic coating is provided on theembedded article.

In the practice of the present invention, the operating equipmentdescribed in U.S. Patent 3,079,276 can be effectively employed. Forexample, a charge of finely divided aluminum, finely divided chromium,finely divided silicon carbide and finely divided halide carrier isblended and placed in a heat-resistant retort and the articles to becoated are cleaned, if necessary, and embedded in the charge.

Table I shows the compositional ranges, and sizing for the charge inaccordance with the present invention and Table II lists exemplarycompositions which have been coated by the process of the presentinvention. Suitable carrier materials are ammonium halides, aluminumhalides and nickel halides.

TABLE I.COMPOSITION OF CHARGE MATERIAL, WEIGHT PERCENT Broad RangePreferred Range Optimum Charge Material Type Particle Particle ParticleSize U.S. Content, Size US. Content, Size U.S. Content, Screen PercentScreen Percent Screen Percent Mesh Mesh Mesh Atomized metal powder -1004-10 200 4-8 200 5 Electrolytic metal powder 100 -30 200 10-20 250Commercially available 58-86 150 71-85 -240 79. 4 6-1 grade 0. 2-1 50 0.2-0. 5 0. 6

applications, including gas turbine and oil refinery equipment, articlescoated by the processes of the aforedescribed patents and patentapplication, were attacked when exposed to sulfur-containing media atelevated temperatures. Consequently, in view of the superior performanceof diffused metallic coatings generally, e.g. oxidation resistance, itwould be of significant industrial benefit if dilfused coatings could beprovided which are resistant to chemical corrosion, particularly insulfur-containing media at elevated temperatures.

It is therefore an object of the present invention to provide avapor-diffusion coating for metallic articles which is resistant toattack in sulfur-containing media at elevated temperatures.

It is another object of the present invention to pro vide a vapordiffusion coating for metallic articles which is strong and oxidationresistant in addition to being resistant to corrosion insulfur-containing media.

Other objects will be apparent from the following de scription andclaims taken in conjunction with the drawing in which:

FIGURES 1 and 4 respectively show an uncoated metal article before andafter exposure to sulfur-containing media at elevated temperatures;

FIGURES 2 and 5 respectively show metal articles, coated by a commercialprocess, before and after exposure to sulfur-containing media atelevated temperatures and FIGURES 3 and 6 respectively show metalarticles coated by the process of the present invention before and afterexposure to sulfur-containing media at elevated temperatures.

The original magnification of the photographs of FIG- URES 1 through 6is 400x.

TABLE II.-IYPICAL ALLOYS COATED BY THE PROCESS OF THIS INVENTION[Nominal compositions, in weight percent] I May also contain othermodifying elements such as: carbon, silicon, boron, zirconium, titanium,copper.

2 Contains thoria dispersed within the nickel matrix.

After the charge and articles to be coated are in place, and with airexhausted from the retort, the retort is suitably sealed and thecontents are heated at a temperature between about 1850 F. and 2l0 0 F.,preferably 2000 F. for a time sufiicient to provide the desired coatingthickness. A suitable process operating time is 5 to 20 hours with 7hours at 2000 F. being preferred.

After the process has operated for a time sufficient to provide thedesired coating, the retort is cooled to about 200 F. or lower and thecoated aritcles recovered and cleaned, e.g., with a bristle brush toremove any loose charge material.

The coated articles thus prepared have smooth uniform coatingscontaining chromium, aluminum and silicon and are resistant to attackfrom sulfur-containing media. Also, the coated articles have excellentoxidation resistance and very good mechanical properties as hereinaftermore specifically indicated.

The following Example I will more fully illustrate the presentinvention.

EXAMPLE I An article in the form of cast disk inch in diameter by inchthick having a composition corresponding to alloy N-l of Table II (13%Cr, 4.5% Mo, Al, 2.5% Cb-t-Ta, bal. Ni) was lightly grit blasted, washedwith water and thoroughly dried. A layer about two inches thick of thepreferred charge material shown in Table I (5% aluminum, 15% chromium,79.4% silicon carbide, 0.4% ammonium bromide, 0.2% aluminum chloride)was placed in the bottom of a retort of the type disclosed in U.S.Patent 3,079,276 and the article to be coated was placed on this layerand completely covered with additional charge. The retort was thencovered and sealed with a fusible silicate material and the retort andcharge were heated to 2000 F. During heating of the charge totemperature, air was driven from the retort and passed out through thefusible silicate seal. The retort and charge were maintained at 2000 F.for 7 hours after which they were cooled to 200 F. and the metal articleremoved, cleaned with a bristle brush and washed with dilute ammoniumhydroxide solution.

The resultant article had a smooth, uniform chromiumaluminum-siliconcoating about 3 mils thick.

The above procedure was repeated with articles in the form of castingsand wrought articles having compositions corresponding to N6, C-4 andC-1 as shown in Table 1. In all instances, the articles were providedwith a smooth, uniform chromium-aluminum-silicon-containing coating.Coating thickness tended to vary depending upon composition of thespecific alloys being coated. For example, on cobalt base alloys therewas a 1 to 3 mil coating; on nickel base alloys there was a 2 to 4 milcoating and on iron base alloys there was a 1 to 10 mil coating.

To determine the suitability of the coated articles for industrialapplications various tests were conducted as follows:

Resistance to sulfur attack The article to be tested was suspended abouthalf immersed for one hour at 1650 F. in a molten salt solution of 99.5%sodium sulfate, 0.5% sodium chloride. The ex- 4 tent of sulfidationattack on the test article was recorded as weight change in milligramsper square centimeter. Table III shows the comparative results obtainedfor coated and uncoated specimens.

1 Severely attacked, resulting in an adherent layer of corrosion product60 Oxidation test The article to be tested was mounted on a hub whichwas rotated at 1725 r.p.m. and which traveled between a furnace (2100F.) and a water spray quenching station at preset intervals. Sixtyseconds exposure in the furnace and 90 seconds cooling in the waterspray constituted one cycle and the test specimens were subjected todifferent numbers of cycles as shown in Table IV. Oxidation resistancewas measured and recorded as average depth of intergranular attack andweight loss.

Alloy C-4, an excellent high temperature engineering material, but whichis generally not employed in oxidizing environments, was additionallytested by exposure to flowing air at 2000 F. for extended periods. Theresults, in terms of weight change are shown in Table IVa.

TABLE IV.-*OXIDATION TESTS AT 300 CYCLES Average Average Weight Depth ofAlloy Condition Loss in Intcrgrauular Milligrams Attack of Base Metal 523 mils. 36 Nil. 169 3 mils.

53 Nil.

UncoatetL 1, 371 Over 3 mils. Coated. 47 Nil.

Uncoatcd. 65 3 mils.

.l Coated. +31 (gain) Nil.

TABLE IVa.OXIDATION TESTS AT 2,000 F.

Weight Change in rug/cm.

Alloy Condition 4 8 16 32 6 1 100 hrs hrs. hrs. hrs. hrs. hrs.

C 4 Coated +.6 +1.2 +1.8 +2.1 +1.2 .3 C-4 Uncoatetlqi. +1.2 3.99 Severeoxidation Mechanical properties Tensile strength and stress rupturetests were conducted following standard techniques and the results aretabulated in Tables V and VI. While it is generally considered that themechanical properties of alloys are substantially lessened by diffusioncoating processing, it can be seen that articles coated by the presentinvention have very high strength.

TABLE V.-SHORT-TIME TENSILE DATA [Articles coated by the process of thisinvention (average of 3 tests, as-cast specimens)] Ultimate Yield Reduc-Temp., Tensile Strength Elongation in Alloy Condition F. Strength, at0.2% tion, Area,

p.s.i. Offset, Percent Percent p.s.i.

Room 147, 200 121, 600 9 8 1, 300 132, 600 102, 8 9 Room 123, 100 106,800 4 4 1, 500 114, 500 88, 200 10 16 Room 102, 900 66, 400 10 12 Room113, 300 74, 700 3 4 1, 050 87, 800 47, 800 l9 l8 1, 050 98, 600 43, 5000 9 C-4 Coated Room 103, 800 82, 100 4 5 TABLE VI.STRESS-RUPTURE DATAAlloy Test 'Iem- Stress, p.s.i. Life, Hours Elongation,

perature, F. Percent Fatigue A Krouse Rotating Beam Testing Machine wasused under standard operating conditions for this test. The number ofrepeated reversed bending stresses to which the specimen was subjectedat high speed until failure were measured and recorded. The averageresults are shown in Table VII.

TABLE VII Alloy N-l Stress p.s.i 25,000 Rotating speed, r.p.m. 8,000Average cycles to failure:

Coated this invention 1 300,000,000 Uncoated as cast 11,540,150

1 Testing discontinued, no failures.

To further demonstrate the improved industrial utility of the presentinvention, alloys N-l, N-6 and C-4 in the form of cast disks inch indiameter by A inch thick were coated following the procedure ofcopending patent application 317,833, now U.S. Patent No. 3,325,305referred to herein as PC1. This coating procedure comprised embeddingthe article in a mixture of 49.5 percent prealloyed 70% Fe30% Almaterial, 49.5 percent silicon carbide, 0.8 percent ammonium bromide and0.2 percent aluminum chloride. The prealloyed material was prepared asdescribed in U.S. Patent No. 3,079,276 and was particulated to aboutminus 20 mesh.

The silicon carbide was also particulated to about minus 60 mesh. Thetwo halide carrier materials which were used were commercially availableCP grade chemicals particulated to about minus 100 mesh and finer. Thefour ingredients were mixed in a twin-cone-type blender for about onehour. The alloy article was lightly grit blasted prior to embedding inthe mixture and placed in a retort as described in US. Patent No.3,079,276 in such a manner that it was completely embedded in theabove-mentioned mixture.

The retort was then placed in a furnace and heated to about 1700 F. for18 hours with the exclusion of air. After the heating step, the retortwas permitted to cool to below about 200 F. and the article was removedfrom the retort and brushed with a soft bristile brush to remove anyadhering particles of the mixture. The coated article was then washed ina dilute ammonium hydroxide solution and rinsed in clear water anddried.

Articles produced in the foregoing manner were then tested forresistance to sulfur attack in the same manner as before described,i.e., suspended half-immersed for one hour at 1650 F. in a molten saltsolution of 99.5% sodium sulfate, 0.5% sodium chloride. The results ofthe test together with data from Table III relating to coated articlesof the present invention are shown in Table VIII. A comparison of thedata of Table VIII shows the superiority of articles of the presentinvention with respect to sulfidation attack.

TABLE VIII.SULFIDATION ATTACK ON SELECTED SPE CIMEN S Weight Change inmg./cm.

Coated by the Alloy No. Base Metal Process of- Uncoated Specimens PC-lThis Invention 1 Severely attacked resulting in adherent layer ofcorrosion product as shown in Figure 4.

In connection with Table VIII reference is taken to the photographs ofthe drawing wherein FIGURE 1 shows an uncoated N-1 alloy specimen andFIGURE 4 shows the same specimen after being subjected to the previouslydescribed sulfidation attack test. The dark grey area 10 in FIGURE 4indicates the corrosion product formed on the uncoated metal and thelight grey area 11 indicates sub-surface'attack. FIGURE 2 shows aspecimen of N1 alloy coated according to the PC1 technique and FIG- URE5 shows a similarly coated specimen after being subjected to thedescribed sulfidation attack test. The numeral 12 in FIGURE 2 indicatesthe original coating depth and 12 in FIGURE 5 indicates the coatingremaining after the sulfidation attack test. FIGURE 3 shows a specimenof N-l alloy coated by the procedure of Example I of this invention andFIGURE 6 shows a similarly coated specimen after being subjected to thesame sulfidation attack test. The numeral 14 in FIGURE 3 indicates theoriginal coating depth and 14 in FIGURE 6 shows the coating after thetest.

As can be seen, from Table VIII and the photographs of FIGURES 1 through6, the material coated in accordance with the present invention providesvery superior resistance to sulfidation attack. Also, as previouslynoted and as shown in Tables IV and Wu, articles coated in accordancewith the present invention are provided with excellent oxidationresistance and Tables V and VI show that articles coated in accordancewith the present invention have very good mechanical properties. TableVII additionally clearly shows that coated articles in accordance withthe present invention have superior resistance to fatigue.

With particular regard to the practice of the process of the presentinvention it has been found that the composition ranges for the chargemixture as set forth in Table I are critical in that variations fromthese proportions result in unsuitable coatings. It was found that aratio of about 3 to 1, chromium to aluminum, is preferred for optimumresults. Elemental chromium (at least 99% Cr) in the form of fine powderpreferably about minus 200 mesh, and elemental aluminum (at least 99%Al) in the form of fine powder, preferably about minus 200 mesh, asavailable commercially, are required instead of prealloyed chargematerials, in order to obtain smooth uniform coatings. Also, the use ofinert fillers and as A1 0 clays and silica in place of silicon carbideis not suitable in the practice of the present invention and suchmaterials are to be avoided since they cause the formation of roughsurfaces and caking of the charge. Further, silicon carbide is requiredas a charge constituent in the present process so that the coatingproduced will contain silicon, ranging from about 0.2 to about 1%, whichhas been found to improve the properties of the coating by stabilizingthe same and thus retarding diffusion of the coating into the substratemetal during subsequent high temperature use. Particulated commerciallypure silicon carbide, preferably minus 200 mesh, is availablecommercially.

Table IX shows, by way of specific examples, the criticality of theprocessing conditions of the present invention.

TABLE IX.--DIFFUSION COATING TESTING RESULTS Compounds as Carrier Agents Charge Material Mixture, Weight Percent Containing Halide NiAI FeCr

Heating Remarks Time, Temper-a A110 1 Hours i ture, F.

18 1. 850 Rough surface; mixture caked.

30 18 1, 8') .i 0. 35 18 1. 850 Rough surface; adherent particles. 321?; 1. 850 Rough surface; mixture cakes. 33 18 i 1.850 Rough surface. 171, 750 Mixture coating caked severely. 17 1, 750 Mixture caked; roughcoating surface;

hardened. 43 16 1,900 Mixture caked; rough coating surface.

7 2. 000 N0 caking; smooth coating surface. 16 1, 900 No coking; smoothsurface. 16 1. 900 Do. 1? 1, 750 DO.

A further advantage of the present invention is that the processoperating temperatures coincide with the temperatnre generally requiredin the heat treatment of cohalt-base alloys and it has been found thatthe heat treatment and diffusion coating can be performedsimultaneously.

While the foregoing specification has been directed to the difi'usioncoating of nickel base, cobalt base and iron base alloys, other metallicsubstrates which are capable of being diffusion coated at temperaturesof l8502l00 F. can be processed in the manner disclosed. For example,cast and wrought articles have been successfully coated. Further, coppermay be coated with the process of this invention at a temperaturebetween 1150 and 1450 F. for 2 to 6 hours.

Mesh sizes referred to herein are United States Sieve Series.

What is claimed is:

1. A method for diiiusion coating metallic articles comprising:

(1) embedding the article to be coated in a particulated charge materialconsisting essentially of about 4 to 10 percent elemental aluminum,about 10 to 30 percent elemental chromium, about 58 to 86 percentsilicon carbide and about 0.2 to 1 percent of a halide carrier material,and

(2) heating the charge material and embedded article with the exclusionof air at an elevated temperature for a time sufficient to form analuminum-chromiumsilicon-containing coating.

2. A method for diffusion coating metallic articles comprising:

(1) embedding the article to be coated in a particulated charge materialconsisting essentially of 4 to 8 percent elemental aluminum, 10 to 20percent elemental chromium, 71 to 85 percent silicon carbide and 0.2 to0.5 percent of a halide carrier material.

(2) heating the charge material and embedded article with the exclusionof air at a temperature between 1850 F. and 2100 F. for about 5 to 20hours to form an aluminurn-chrornium-silicon-containing coat- 3-. Amethod for diffusion coating metallic articles comprising:

(1) embedding the article to be coated in a particulated charge materialconsisting essentially of about 5 percent elemontal aluminum, about 15percent elemental chromium, about 79 percent silicon carbide and about0.6 percent of a halide carrier material (2) heating the charge materialand embedded article with the exclusion of air at a temperature of about2000 F. for about 7 hours to form an aluminumchromium-silicon-containing coating.

4. A metallic article having a protective aluminumchromium-silicon-containing coating formed by heating the articie withthe exclusion of air at a temperature between about 1850 F. and 2150 F.in a particulatcd charge material consisting essentially of 4 to 10percent elemental aluminum, 10 to 30 percent elemental chromium, about58 to 86 percent silicon carbide and 0.2 to 1 percent of a halidecarrier material.

References Cited UNITED STATES PATENTS 3,095,316 6/1963 Hartwig117--107.2 3,254,969 6/1966 Bungardt ll7-107.2 3,257,230 6/1966 Wachtellet a1. 117107.2 3,298,858 1/1967 Ashikari 117107.2

ALFRED L. LEAVITT, Primary Examiner.

A. GOLIAN, Assistant Examiner.

1. METHOD FOR DIFFUSION COATING METALLIC ARTICLES COMPRISING: (1)EMBEDDING THE ARTICLE TO BE COATED IN A PARTICULATED CHARGE MATERIALCONSISTING ESSENTALLY OF ABOUT 4 TO 10 PERCENT ELEMENTAL ALUMINUM, ABOUT10 TO 30 PERCENT ELEMENTAL CHROMIUM, ABOUT 58 TO 86 PERCENT SILICONCARBIDE AND ABOUT 0.2 TO 1 PERCENT OF A HALIDE CARRIER MATERIAL, AND (2)HEATING THE CHARGE MATERIAL AND EMBEDDED ARTICLE WITH THE EXCLUSION OFAIR AT AN ELEVATED TEMPERATURE FOR A TIME SUFFICIENT TO FORM ANALUMINUM-CHROMIUMSILICON-CONTAINING COATING.